qmake: make message about not found mkspec less confusing

Change-Id: I9cfa2c9476d3d7fdb68b0a23e175eea4b3b11ad9
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
(cherry picked from qtcreator/c0ddf0a34407f9d3ad9f575176550ba33d536d12)
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qmake/library/qmakeevaluator.cpp

@@ -1259,7 +1259,7 @@ bool QMakeEvaluator::loadSpec()
                 goto cool;
             }
         }
-        evalError(fL1S("Could not find qmake configuration file %1.").arg(qmakespec));
+        evalError(fL1S("Could not find qmake spec '%1'.").arg(qmakespec));
         return false;
     }
   cool: